This specification relates to introducing an analyte into a chemical analyzer for analysis.
Chemical analysis tools such as gas chromatographs (“GC”), mass spectrometers (“MS”), ion mobility spectrometers (“IMS”), and various others, are commonly used to identify trace amounts of chemicals, including, for example, chemical warfare agents, explosives, narcotics, toxic industrial chemicals, volatile organic compounds, semi-volatile organic compounds, hydrocarbons, airborne contaminants, herbicides, pesticides, and various other hazardous contaminant emissions. A summary of available detection technologies is contained in Yin Sun and Kowk Y Ong, Detection Technologies for Chemical Warfare Agents and Toxic Vapors, 2005, CRC Press, ISBN 1-56670-668-8 (“Sun & Ong”).
Chemical detectors have a minimum concentration of analyte in a matrix that can be detected. For some chemicals, particularly threats, it is desirable to detect at extremely low concentrations compared to the sensitivity limit of typical instruments. For example, in some uses, instruments must be capable of detecting chemicals present to at or below 1 ppb to be effective. Table 1, below, is adapted from shows the Immediate Danger to Life and Health (IDLH) values for several common Chemical Warfare Agents (CWAs). As can be seen from examination of this table, these agents are dangerous at concentrations down to 2 ppb, hence instruments intended to detect various CWAs must be able to detect below the corresponding IDLH.
TABLE 1IDLH values of common CWAs. Adapted from Sun & OngCWACASIDLH (ppm)GA71-86-60.030GB107-44-80.030GD96-64-00.008GF329-99-70.030VX50782-69-90.002
Further, many explosives have very low volatility indexes and as such, emit a very low amount of vapor into the surrounding air. In the case of mass spectrometers, which typically require that the chemical sample be introduced into the instrument in a gaseous form, low sensitivity limits would be particularly useful. In particular, for mass spectrometers to effectively detect the presence of explosives simply by analyzing the air in the proximity of the instrument, extremely low sensitivity limits are desirable (ideally parts per trillion).
To facilitate this low concentration detection, some systems include a chemical pre-concentrator to increase the apparent concentration of samples being introduced to the chemical analyzer. For example, the apparent concentration of a sample introduced into an analyzer can be increased by using a membrane between the sample inlet and the chemical analyzer to remove or block certain species, while allowing target species to flow into the analyzer. While membrane inlets have been proven effective in commercial applications, they are typically limited to small concentration gains (<100) and are selective in the types of materials that are allowed through the membrane. An alternative approach is to use solid sorbent tubes to trap the species of interest. Conventional sorbent tubes are typically composed of a metal or glass tube packed with glass fibers or beads coated with or comprised of absorptive material, solid absorbent (e.g., calcium chloride, silica gel), or a variety of sorbent materials suited for the particular application. It should be noted that the terms absorption (implying an interaction of the analyte with the bulk material) and adsorption (implying an interaction with the surface of a material) are both used interchangeably. The specific mechanism of collecting the analyte is material dependent and all forms of collection are covered by the scope of this disclosure. The tubing is typically wrapped in Nichrome wire which heats the tubing when an electrical current is passed through it. During the collection phase, a sample is passed (e.g., by carrier gas, or liquid) through the tube while the sorbent material sorbs the analyte. These sorbents are then heated, releasing the analyte into the analyzer in a much shorter time than they were sorbed, thus increasing the concentration “seen” by the chemical analyzer.
Indirectly heating the sorbent material often results in various inefficiencies. For example, the sorbent material within the tube typically provides poor heat conduction paths, thus hindering the heat flow to the interior of the tube. Further, additional power and time is typically required to compensate for the loss of heat into the surroundings. In addition, the sorbent material often impedes the passage of the carrier gas during sampling and desorption. Still further, while large gains in concentration are possible, conventional sorbent tubes may have other drawbacks: 1) there can be a substantial amount of time and power required to sorb & desorb sufficient material, 2) the various locations on the sorbent material are not heated simultaneously thus releasing analyte at different times; hence reducing the apparent concentration seen at any one sample time and broadening the overall resolution of the pre-concentrator, 3) reactions between the analyte, sorbent, and background matrix can skew measurements by introducing unknowns into the chemical analyzer, 4) they can be very selective in that the gain measured between different sorbents can vary dramatically, 5) the sorbent material is not heated uniformly thus some analytes will be released at different times and to varying extents.